linux-kernel - Re: [PATCH v8 18/19] swiotlb-xen: no need to do execute two bus_to

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Message-ID: <alpine.DEB.2.02.1310231833560.26077@kaball.uk.xensource.com>
Date:	Wed, 23 Oct 2013 18:34:13 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	<xen-devel@...ts.xensource.com>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <Ian.Campbell@...rix.com>
Subject: Re: [PATCH v8 18/19] swiotlb-xen: no need to do execute two bus_to_phys
 translations in a row

> On Thu, Oct 17, 2013 at 06:43:33PM +0100, Stefano Stabellini wrote:
> > xen_unmap_single calls xen_bus_to_phys, then passes dev_addr to
> > is_xen_swiotlb_buffer that calls xen_bus_to_phys again.
> 
> It does?
> 
> I see it call mfn_to_local_pfn which calls pfn_to_mfn and
> get_phys_to_machine (which I presume is going to be a slow
> operation on ARM).

Yeah, I am trying to reduce the number of pfn_to_mfn and mfn_to_pfn
translations.
That said, this patch is not needed to have a fully functional
swiotlb-xen on arm and I would OK with postponing it.


> > xen_bus_to_phys is potentially a slow operation that might involve
> > walking a tree.
> > We can avoid calling xen_bus_to_phys twice by removing the
> > is_xen_swiotlb_buffer call and doing the check directly in
> > xen_unmap_single using the physical address.
> > 
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
> 
> Have you tested this with a PFN that is in a foreign domain?

That would be blkback. I only tested blkback on ARM, I'll test on x86
too.


> I am bit uncomfortable with this - I recall when developing this
> that I had hit some odd cases of doing DMA of a page to a guest
> and it not always working. It was the case of not detecting whether
> the pfn was foreign or not. Sadly I can't recall the details.
> 
> I would think that this problem would also be present with ARM?
> But perhaps all the new changes in the pfn_to_mfn that had lately
> been done have fixed this?

No, because on ARM the guest is autotranslate so the pfn returned by
mfn_to_pfn is always "local".
However thanks to the m2p_override now we should get the "local" pfn
from mfn_to_pfn on x86 too.
I'll test this case though.


> > ---
> >  drivers/xen/swiotlb-xen.c |    4 ++--
> >  1 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> > index 4d50058..6d9ca61 100644
> > --- a/drivers/xen/swiotlb-xen.c
> > +++ b/drivers/xen/swiotlb-xen.c
> > @@ -426,8 +426,8 @@ static void xen_unmap_single(struct device *hwdev, dma_addr_t dev_addr,
> >  
> >  	xen_dma_unmap_page(hwdev, paddr, size, dir, attrs);
> >  
> > -	/* NOTE: We use dev_addr here, not paddr! */
> > -	if (is_xen_swiotlb_buffer(dev_addr)) {
> > +	if (paddr >= virt_to_phys(xen_io_tlb_start) &&
> > +		paddr < virt_to_phys(xen_io_tlb_end)) {
> >  		swiotlb_tbl_unmap_single(hwdev, paddr, size, dir);
> >  		return;
> >  	}
> > -- 
> > 1.7.2.5
> > 
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